Answer
Radar is short for ra(dio) d(etecting a(nd) r(anging). Radar works by emitting
pulses of electromagnetic waves toward a target and detecting a small portion of
those waves that are reflected back to the receiving antenna. The transmission and
reception is usually done by the same dish-shaped antenna. The time between
transmission and reception is used to calculate the distance to the target by
multiplying one-half the round-trip time by the speed of light to get an answer in
terms of length.

Sonar or so(und) na(vigation) r(anging) works in a manner similar to radar, except
sonar uses pulses of sound waves underwater to find the distance to a
sound-reflecting target. Since the speed of sound is about 196,000 times slower
than light(in sea water), the response time for sonar is proportionately longer.

Why not use radar underwater? The catch is that radar uses radio waves in the
microwave frequency range, or approximately one centimeter in wavelength. This
wavelength range is used because it is easier to direct the waves with small
antennas in narrow beams. Unfortunately, Microwaves are strongly absorbed by sea
water within feet of their transmission. This renders radar unusable underwater.
Answered by:
Scott Wilber, None, President, ComScire - Quantum World Corporation

The reason is mainly because radar has a harder time penetrating large volumes of
water. Contacts made by submarines are often dozens of miles away, and radar would
have to be EXTREMELY powerful to reach that far in water, while sound (a mechanical
wave) can make it that far.

Also, radar is only an active system allowing for your detection by passive
sensors. Whereas sonar can be both passive and active. You can listen to sounds
made by other subs' propulsion without giving away your position. That is very
important in Anti-Submarine Warfare, and is why quieter submarines have an edge on
survivability.

Then there's the prospects of electromagnetic jamming. Sonar jamming wouldn't be
practical, because it would reveal your position and wouldn't mask your propulsion
signature very well. The only way to mask your engine using audio countermeasures
would be to generate a negative waveform of your signature and produce it in real
time, which is very hard and also impractical.

So far, this has been because of military applications, but what about civilian
use?

Sonar is much cheaper and works just as well for their purpose of mapping the ocean
floor or whatever they do with it.

Also, there is not really much advantage to having a faster wave. Sound travels
very fast in water compared to air, so you can hear things at distance in near
real-time. It is also easier to measure the doppler effect with sound than radio.

Radar is just too impractical to use underwater.
Answered by:
Justin Clifford, High School Student, Alpine, Utah